1. Field of the Invention
This invention relates to electrical motors and, more particularly, to commutators for the rotors of electric motors in which positive electrical contact between the rotating commutator and brushes secured to the stationary stator is made throughout substantially three hundred sixty degrees of rotation of the commutator.
2. Description of the Prior Art
An electric motor, or generator, includes two major portions, a movable portion, generally referred to as the rotor, or armature, and a stationary portion, generally referred to as the stator. The generation of electricity, or the utilization of electricity depends on the relative motion between the armature and the stator.
The commutator is the part of the armature to which the armature coils of a motor are connected. It generally comprises a plurality of wedge shaped copper segments or bars disposed about a central steel hub or shaft. The segments comprising the commutator are insulated from each other and from the hub. The brushes ride on the outer peripheral edges of the commutator bars to connect the armature coils to a power source. The commutator segments extend radially outwardly from the center hub, which is part of the shaft of a motor or generator. The commutator segments are insulated from each other to provide polarity control for the armature.
The brushes are typically made of carbon, which is a considerably softer material than the copper out of which the commutator is made. In order to maintain a positive contact between the commutator and the brushes, a spring bias is applied to the brushes to hold them against the commutator. Accordingly, there is wear in the brushes occasioned by the frictional contact over a period of time.
In addition to the frictional wear of the brushes, the brushes and the commutator are subject to wear by arcing between the commutator and the brushes as the brushes pass over the insulated gap between the commutator bar. The arching is caused by the momentary break in electrical contact between the brushes and the commutator bars as current continues to flow and flows across the gap.
The arcing represents wasted energy as well as a wear factor on the brushes. Furthermore, the arcing must be controlled, or contained, in certain environmental applications of an electrical motor where the arcing could result in explosions or fires.
Generally speaking, the higher the inductance of the motor, the greater the arcing. Accordingly, with motors of high inductance, the arcing tends to be more of a problem than with motors of low inductance.
Prior art methods of eliminating or reducing the arcing includes the employment of diodes, such as disclosed in U.S. Pat. No. 3,381,210. A diode is used in the '210 patent to connect the slip rings on the rotor shaft through which current is supplied to the field winding of the alternator. The diode is polarized to provide a return path for energy stored in the field winding when the current flow to the field winding is interrupted.
A different approach to improving the efficiency of commutators and brushes is shown in U.S. Pat. No. 3,437,898. In the '898 patent, disc commutators are secured to the armature, with thyristors included in the electrical circuitry with disc commutators and brushes.
U.S. Pat. No. 3,532,949 discloses another approach to reducing or eliminating sparking between a commutator and its brushes. Diodes and thyristors are used to polarize and control the current flow to prevent the arcing. Aside from the differences in the circuitry, the commutator and brush system of the '949 patent is typical of the prior art commutators and brushes, as described above. Similarly, the commutator and brush arrangement is typical of the prior art.
The apparatus of the present invention employs a different type of commutator than is disclosed in the prior art.